I’ve moved your post to the #roon category rather than the #roon-community-site category since you are asking (in effect) what would be best to run the Roon software…
A couple of thoughts, @danny claims that Roon Labs have sold thousands of Nucleus units since their introduction, so failure rates are low. Secondly, if you don’t want to buy a Nucleus, why not build your own, using an Intel NUC and ROCK?